4D Fetal Echocardiography

Congenital heart defects (CHD) are among the most common birth defects, occurring in 5 to 10 per 1000 live births. This substantial variation in the reported epidemiology of CHD is due to differences in applied methodologies. An increasing total prevalence of CHD has been recently reported, mainly due to increase in prevalence of small defects easily diagnosed by echocardiography, as well as an increase in prevalence of conotruncal defects and atrioventricular septal defects.In order to provide a comprehensive epidemiological overview, future studies should use international classification system as well as consistent inclusion and exclusion criteria. Futher studies are also required to evaluate precisely the impact of fetal cardiac diagnosis on the prevalence and outcome of CHD. Epidemiology of CHD provides an overview of the distribution and characteristics of risk factors. Environmental potential risk factors are reviewed as they may provide an opportunity for prevention of some forms of CHD.

Congenital heart diseases (CHD) are frequent fetal anomalies, with an important impact on perinatal mortality and morbidity. Prenatal diagnosis has a demonstrated effect in decreasing the overall prevalence at birth, and in improving the outcomes of specific malformations; moreover, the great majority of these malformations occurs in the absence of any risk factor. All of these features strongly evidence the need for a screening test that can be applied to the general population, to identify a selected group of fetuses for the more specific, complex, and time-consuming diagnostic test (fetal echocardiography). The systematic visualization of the four chamber view and the outflow tracts during routine anatomical scan has progressively increased the prenatal detection rate of CHD. However, a number of potentially diagnosable defects are still missed in the screening setting. Three-dimensional ultrasonography could provide a useful tool to ameliorate the current performance of CHD screening; however this possibility needs to be further explored.

The heart, anatomically and embryologically, is a segmented structure. The atria, the ventricles and the great arteries are the three major fundamental components. The heart should be examined echocardiographically by Sequential Segmental Analysis from the venous to the arterial poles following the blood flow. Each segment is evaluated independently considering not only the segmental situs (or location into the body) but also the connections of each heart segment to the others. A systematic approach by Sequential Segmental Analysis is a cornerstone of fetal cardiac study. First step is the identification of fetal heart position followed by the identification of position of the heart in relation to the body and the anatomic study of each cardiac chamber, and finally, the study of cardiac rhythms and function. The heart can be observed in infinity of planes, but few sections are the basis of fetal cardiac study. The examination starts with abdominal cross-sectional view for the identification of the viscero-atrial situs. Then the transthoracic four-chamber view should be obtained. This view allows to obtains a large amount of informations specially regarding the atria and ventricles, atrioventricular valves, interatrial and interventricular septum. Ventriculo-arterial connections are well identified by a gentle sweep towards the fetal neck, left ventricular outflow tract and ascending aorta are first visualized and, by further sweeping, also right ventricular outflow tract, branch pulmonary arteries and ductus arteriosus are correctly identified. The three vessel view give us information regarding left or right location of aortic arch. Short axis of the heart is evaluated from the cavo-atrial and ventricular segments to the ductal and aortic arches. Finally the study of heart rate and rhythm and of myocardial function ends the fetal heart examination.

Three dimensional (3D) echocardiography represents a major paradigm shift in medical ultrasound imaging. As opposed to reconstructed 3D echocardiography which is based on a series of two dimensional images, the revolutionary development of the two dimensional (2D) matrix phased array transducer permits true real time live 3D imaging of the fetal heart. Preliminary experience has demonstrated that real time/live 3D echocardiography is not only feasible in assessing the fetal cardiac anomalies, it also permits unique comprehensive views of the fetal heart that are unattainable with 2D echocardiography. With further evolution of the technology, and appropriate education and training RT3D echocardiography may evolve as a powerful supplement to conventional 2D echocardiography.

This chapter summarizes different approaches currently used to examine the fetal heart using three and four-dimensional ultrasonography with a particular emphasis on multiplanar display and novel rendering modalities. These new imaging modalities provide important insight into the normal and abnormal fetal cardiac anatomy and function and have the potential to reduce the operator dependency that characterizes twodimensional ultrasound.

The purpose of this paper is to review how 3D/4D echocardiographic images can be manipulated to maximize the information regarding cardiac structure to improve the diagnostic yield of congenital heart defects. The following topics are discussed: (1) the differences between 3D and 4D ultrasound; (2) basic imaging principles of fourdimensional ultrasound when using spatial-temporal image correlation, real-time volume display, and matrix array technology; (3) analysis of acquired volumes using X,Y and Z rotation, the transverse sweep, the spin technique, tomographic imaging, rendering, inversion, and B-flow. The conclusion is that while the 3D and 4D volumes are easy to acquire, the user must understand the number of tools available to enhance the examination of the fetal heart.

High Definition Power Flow Doppler (HDPD) is a bi-directional power Doppler imaging mode. B-flow is a direct-volume non-gated scanning tool that images blood flow in real-time. These ultrasound modalities have improved images of the fetal cardiovascular system to near-angiographic quality. Each has its particular advantages, which are illustrated with examples of fetal cardiovascular malformations obtained with these tools. HDPD and B-flow provide images not available to practitioners even five years ago.

Real-time four-dimensional (4D) echocardiography is the examination of the fetal heart in the three spatial dimensions plus motion.The matrix probe’s technology allows direct volume scanning by electronically interrogation of a region of interest and acquisition of a pyramidal volume of ultrasonographic data. This technology has the potential to minimize motion artifacts associated with 3D/4D ultrasonography with a satisfactory spatial resolution.The system allows beam steering and focusing in the 3D volume dataset, making it possible to simultaneously examine two different planes of section of the same structure, in real-time, without resolution loss (Live xPlane imaging). The system achieves a pyramidal volume of data, creating a new real-time 3D moving imaging mode called Live 3D Volume Imaging, without the use of software-reconstructed section planes. Combination with Doppler techniques creates a new imaging option: Full Volume 3D imaging. The technique of Thick Slice Live Volume Imaging allows high resolution and contrast-enhanced images The strength of this technology is the easiness in obtaining real-time images of the heart, the high volumetric frame rate and the opportunity to obtain scanning planes with the same axial and lateral resolution. These features improve the overall understanding of anatomical structure arrangement. Matrix technology could be very useful in congenital cardiac clinical applications: real-time 3D echocardiography shows instantaneous rendered images of the beating fetal heart with complex pathologies in one complete heart cycle, and application of this modality allows its spatial location in the heart and its temporal location in the cardiac cycle.

The recent introduction of 3D/4D ultrasonography to clinical practice presents an important milestone in imaging technology. In this chapter, we will present the basic principles of 3D automated sonography as it relates to the examination of the fetal heart. This approach holds great promise for simplifying the fetal cardiac examination that may translate to increased prenatal detection of congenital heart disease.

Abnormalities of systemic and pulmonary venous connections are among the most frequently missed congenital heart disease (CHD) in prenatal ultrasound studies. In fact their prenatal US detection is difficult and requires adequate image resolution and attention to detail.

Recently, three-four D US has been suggested to provide a significant contribution to our understanding of the developing heart in both normal and anomalous cases. In particular “B-flow-STIC imaging” and “inversion mode” have been demonstrated to supply additional information over that provided by 2D US in the prenatal diagnosis of some congenital heart defects, including abnormalities of the venous connections.

In this chapter we report the 2D prenatal characterization of the most common anomalies of the venous connections, and describe the application and added value of 4D echocardiography with B-flow-STIC imaging or with inversion mode in the prenatal diagnosis of total anomalous pulmonary venous connections and abnormal systemic venous connections respectively.

Atrial, ventricular and atrioventricular septal defects are the most common cardiac anomalies in the humans and occur isolated or as part of other malformations in more than the half of children with a congenital cardiac anomaly. Whereas atrial defects are difficult to detect antenatally, ventricular and atrioventricular defects are detectable on two-dimensional and color Doppler ultrasound. Three-dimensional ultrasound as STIC technology allows in fetal septal defects on one hand a safe description and documentation of the finding and on the other hand a spatial demonstration of the defect with the possibility of getting new views into the heart. The choice of the ideal plane from a 3D volume enables to get the in-line visualization of the interventricular septum with the septal defect. Orthogonal views help to visualize the defect in the different planes. Tomographic imaging aids in getting the upper abdomen and the great vessels information in addition to the septal defect view, in order to rule out a complex malformation. Rendering mode with the enface view can be used to visualize the septum from a lateral view and the common atrioventricular valve in atrioventricular septal defects. The combination with color Doppler helps to get the spatial demonstration of the defect within the heart and provide in addition information on flow events during the cardiac cycle in septal defects.

In this chapter, the 2D, color Doppler and four dimensional (4D) features of major right heart abnormalities are described. In particular, the echocardiographic views on which the various lesions are present are reported. The diagnostic role of 4D echocardiography in allowing a spatial demonstration of the defects with the possibility of getting new views into the heart is outlined. Videos of major diagnostic features are also provided, to facilitate the understanding of the text

In this chapter, the 2D, color Doppler and four dimensional (4D) features of major right heart abnormalities are described. In particular, the echocardiographic views on which the various lesions are present are reported. The diagnostic role of 4D echocardiography in allowing a spatial demonstration of the defects with the possibility of getting new views into the heart is outlined. Videos of major diagnostic features are also provided, to facilitate the understanding of the text.

In the chapter, the 2D, color Doppler and 4D features of major conotruncal abnormalities will be described. In particular, the echocardiographic views on which the various lesions are detected will be described. In addition, the role of color Doppler in the recognition of valve stenosis or insufficiency will be illustrated. Finally, the diagnostic role of 4D echocardiography will be described, only in those cases in which it has additional clinical value. Videos of major diagnostic features are also provided, to facilitate the understanding of the text.

The following chapter discusses the added value of various novel 3- and 4-Dimensional Ultrasound (3D/4D US) applications for assessing fetal cardiac anomalies at the first trimester of pregnancy. Information on the importance of the early diagnosis, the feasibility, the anomalies that can be detected at early stages of pregnancy and the limitation of such early examination are discussed in details.

Accurate and reliable methods to assess fetal cardiac function would be useful in evaluating fetuses with cardiac disease (structural or otherwise). Traditionally, two-dimensional echocardiography has been used to estimate fetal ventricular volume, and assess cardiac function. However, the unique and complex geometry of the fetal ventricles makes analysis of cardiac function using this modality a challenge, and hence, the interest in using three- and four-dimensional ultrasound. Although theoretically appealing, three-dimensional echocardiography had to overcome several difficulties, including: gating, suboptimal image quality, and lack of real-time observation. Four-dimensional fetal echocardiography is a method to assess ventricular volume and cardiac function, and can overcome many of the pitfalls of conventional methods. Thus, this modality offers an important method for the assessment of fetal cardiac function.

Fetal magnetic resonance imaging (MRI) is a third-level diagnostic tool for the study of fetal malformations and has been applied in the diagnosis and definition of fetal central nervous system (CNS) and other fetal, placental and uterine diseases. Recent developments of new realtime sequences during free breathing without cardiac triggering have established a potential role of MRI in the study of fetal heart: MRI can study the morphology using steady-state free precession (TrueFISP) sequences on sagittal, coronal and axial planes, orthogonally oriented to the fetal diaphragm and allows to identify the viscero-atrial situs, the heart and its axis. It is also possible to perform a dynamic study, through the acquisition of cine-MR sequences with real-time steadystate free precession (SSFP) oriented according to the standard projections used in fetal echocardiographic scannings. At the moment, there is no evidence that short-term exposure to electromagnetic fields of 1.5 T or less harms the fetus. MRI can analyze the normal anatomy by transverse, long axis and angulated views to visualize the principal cardiac planes. There are recent evidences of a useful role of MRI in the definition of congenital heart disease (CHD). The study of fetal CHD can be made by direct signs such as volumetric abnormalities of the heart and of the cardiac chambers, abnormalities of the structure, thickness and signal intensity of the myocardial walls, anomalies of the cardiac axis orientation, defects of the ventricular and atrial septa and anomalies of the origin, size and course of the great arteries. The difficulty to recognize a “normal” anatomical structure in the reference projections, the increase of the vascular size before a vascular stenosis and the presence of cardiomegaly and pericardial effusion are instead considered as indirect signs of CHD are considered as suspect for fetal CHD.Despite current limitations, fetal MRI seems to be a promising diagnostic method for the assessment of the fetal heart.

Despite the dramatic improvement in the quality of diagnostic ultrasound Congenital Heart Disease (CHD) are the most common and less frequently diagnosed prenatally anomalies. This is mainly secondary to the difficulties in training expert and dedicated sonographer in the study of fetal heart. Four dimensional (4D) sonography may overcome operator dependency allowing offline 4D examination of the fetal heart. This approach opens the possibility of performing a virtual echocardiography in fetuses at high risk or suspected to be affected by a CHD through networking capabilities. As a consequence virtual echocardiography may allow to extend the benefit of advanced of cardiac examination in patients followed in peripheral centers thus reducing the number of unnecessary referral to tertiary centers.